It is generally known that chromatography involves physicochemical methods for separating mixtures of substances for analytical and preparative purposes, in which the mixture to be separated is distributed by exchange processes on two auxiliary phases and thus fractionated in a way typical of its particular components. The precondition for use of chromatography is, however, that substances present in the mixture can be dissolved or vaporized without alteration. In most chromatographic methods, a liquid or gaseous mobile phase which carries the analysis mixture moves over a solid or liquid stationary phase.
The possibility of using calixarenes for the modification of the stationary support phase for chromatography was mentioned for the first time in the two Japanese published specifications Hei-5-264531 and Hei-6-58920, which propose the modification of support materials composed of glycidyl methacrylate after special preparation of the phases with p-hexachlorosulfonylcalix[6]arene or p-hexaaminocalix[6]arene. However, the phases have the crucial disadvantage that they consist of organic material. These support materials are cost-intensive and, moreover, not resistant to all HPLC solvents. In addition, the preparation of the support phase for the linkage to the particular calixarene is very complicated.
Jeremy D. Glennon et al. partly remedy this disadvantage in Analytical Letters, Jan. 26, 1993, pages 153-162, and in Analytical Proceedings Including Analytical Communications, January 1994, Vol. 33 (1), pages 33-35, since it is proposed to modify silica gel in the stationary support phase with various calix[4]arenes and calix[6]arenes.
In Analytica Chimica Acta, 291 (1994), pages 269-275, Jeremy D. Glennon et al. describe the linkage of the silane of the stationary support phase not directly to the olefinic unit of the calixarene, but uses mercaptopropyltriethoxysilane, which results in a thioether bridge in the molecule. The disadvantage of these solutions is based on the fact that linkage of the triethoxysilyl groups with one another may occur through a partial hydrolysis and subsequent ethanol elimination. This may in certain cases result in bridging of the required pores in the silica gel, which leads to a decrease in the specific surface area of the chromatography phase. Since it is known that only some of the triethoxysilyl groups become attached to the silica gel, hydrolysis results at least in part in terminal trihydroxysilyl groups. However, these are polar and adversely affect the chromatography which is to be operated as reverse phase chromatography. In the case of attachment via the thioether bridge it must, moreover, be expected that the chemical stability of the phase will be reduced.
Another type of attachment to silica gel is described by S. Friebe et al. in Journal of Chromatographic Science, Vol. 33, June 1995, pages 281-284, since they use, to couple on the p-tert-butylcalix[4]arene, a short hydrophilic spacer which, however, is not described in detail in its embodiment.